This invention concerns a phase change material which melts and freezes at a temperature below 32 degrees F. and can be utilized to store coolness in ice bank equipment such as that disclosed in co-applicant Calvin MacCracken's U.S. Pat. Nos. 4,294,078 and 4,403,645. In particular it relates to an improved eutectic composition, and its method of use, having a melting-freezing temperature remaining constant at about 28 degrees F. providing optimum neucleation and minimal precipitation during freezing.
Co-applicant Maria Telkes in her U.S. Pat. Nos. 2,667,664 and 2,989,856 disclosed a phase change material rich in anhydrous sodium sulfate (at least one-third by weight) with a small amount of sodium tetraborate decahydrate (i.e. borax) as a neucleating agent, both added to water. That composition was intended for heat storage rather than coolness storage and had a melting-freezing temperature of about 90 degrees F. The salt was not in the solubility range in water so that a substantial amount of it precipitated upon melting unless a strong gel or thixotropic agent was employed, as described in Telkes' U.S. Pat. No. 3,986,969. The heat of fusion of the Telkes' prior art eutectic was in the order of 103 Btu's per pound and its volumetric storage capacity was approximately 7280 Btu's per cubic foot. Another eutectic of anhydrous sodium sulfate decahydrate mixed with chlorides or potassium nitrate having melting-freezing temperatures above 40 degrees F. for coolness storage is disclosed in Telkes' U.S. Pat. No. 2,989,856.
The object of the present invention is to provide a eutectic composition for coolness storage in ice bank equipment such as that described in the aforementioned MacCracken patent. The eutectic is to have a constant melting-freezing temperature of about 28 degrees F, a heat of fusion substantially higher than that of the prior art, improved solubility in water to reduce precipitation, and optimum nucleation during freezing.